An Experimental Hut Evaluation of PBO-Based and Pyrethroid-Only Nets against the Malaria Vector <i>Anopheles funestus</i> Reveals a Loss of Bed Nets Efficacy Associated with <i>GSTe2</i> Metabolic Resistance

Growing insecticide resistance in malaria vectors is threatening the effectiveness of insecticide-based interventions, including Long Lasting Insecticidal Nets (LLINs). However, the impact of metabolic resistance on the effectiveness of these tools remains poorly characterized. Using experimental hut trials and genotyping of a glutathione S-transferase resistance marker (L119F-<i>GSTe2</i>), we established that GST-mediated resistance is reducing the efficacy of LLINs against <i>Anopheles funestus</i>. Hut trials performed in Cameroon revealed that Piperonyl butoxide (PBO)-based nets induced a significantly higher mortality against pyrethroid resistant <i>An. funestus</i> than pyrethroid-only nets. Blood feeding rate and deterrence were significantly higher in all LLINs than control. Genotyping the L119F-<i>GSTe2</i> mutation revealed that, for permethrin-based nets, 119F-<i>GSTe2</i> resistant mosquitoes have a greater ability to blood feed than susceptible while the opposite effect is observed for deltamethrin-based nets. For Olyset Plus, a significant association with exophily was observed in resistant mosquitoes (OR = 11.7; <i>p</i> < 0.01). Furthermore, <i>GSTe2</i>-resistant mosquitoes (cone assays) significantly survived with PermaNet 2.0 (OR = 2.1; <i>p</i> < 0.01) and PermaNet 3.0 (side) (OR = 30.1; <i>p</i> < 0.001) but not for Olyset Plus. This study shows that the efficacy of PBO-based nets (e.g., blood feeding inhibition) against pyrethroid resistant malaria vectors could be impacted by other mechanisms including GST-mediated metabolic resistance not affected by the synergistic action of PBO. Mosaic LLINs incorporating a GST inhibitor (diethyl maleate) could help improve their efficacy in areas of GST-mediated resistance.